App运行过程中需要更改System Setting（swift）

在用户使用App过程中，比如需要开启定位呀，通知呀，可以在应用内直接打开Setting， 而不用进入后台再一步一步来设置，配合iOS9的新特性（左上角的返回按钮）很方便。

let alertController = UIAlertController(title: "Sad", message: "Please enable it in Settings", preferredStyle: .Alert)let settingAction  = UIAlertAction(title: "Settings", style: .Default) { (settingAction) -> Void in        if let appSettings = NSURL(string: UIApplicationOpenSettingsURLString){UIApplication.sharedApplication().openURL(appSettings)            }        }    alertController.addAction(settingAction)    let cancelAction = UIAlertAction(title: "Cancel",style: .Cancel,handler: nil)    alertController.addAction(cancelAction)    presentViewController(alertController, animated: true, completion: nil)